Bonding composition for ceramics comprising metal oxide melt and method for bonding ceramics

ABSTRACT

A bonding composition comprising a metal oxide melt comprising (a) CaO, (b) SiO 2  and/or Al 2  O 3 , and (c) a metal oxide selected from TiO 2 , ZrO 2 , Cr 2  O 3 , HfO 2 , Nb 2  O 3  and Ta 2  O 5  is melt-bonded on the surface of a ceramic material, and the ceramic material is bonded to an adherent, if necessary through at least one layer selected from a plating layer, a solder layer, and a buffer layer. In this manner, a strong bonding can be very easily obtained.

TECHNICAL FIELD

The present invention relates to a bonding composition for ceramics and a method for bonding ceramics. More particularly, the present invention relates to a bonding composition which can bond silicon nitride (Si₃ N₄) to silicon nitride (Si₃ N₄) or a metal easily and firmly, and a bonding method using this bonding composition.

BACKGROUND ART

Generally, ceramics have excellent heat resistance, corrosion resistance, chemical resistance, hardness, abrasion resistance, and insulating property, and intensive research has been carried out into the use of ceramics as structural parts by utilizing these excellent characteristics.

Silicon nitride (Si₃ N₄) is a material especially suitable for a part for which a high strength and a high abrasion resistance at high temperatures are required, such as a gas turbine rotor or a fan blade liner or for a corrosion-resistant lining member of a chemical reaction tube to be in contact with a molten metal or the like.

But, in the case of ceramics, the integral production of a part having a complicated shape or mechanical processing is still difficult, and since ceramics are inherently brittle materials, the reliability of the strength thereof is inferior to that of metals. Accordingly, the production of products having a complicated shape by bonding ceramic materials or the development of composite structures having both the strength of a metal and the above-mentioned characteristics of a ceramic material by bonding the ceramic material to the metal is desired.

As the method for bonding ceramic materials to each other or bonding a ceramic material to a metal, there are adopted at present a mechanical bonding method such as screwing, shrink fitting or so-called fitting, an adhesive method using an organic or inorganic adhesive, a solder method in which the surface of a ceramic material is metallized and bonding is effected by using a solder, and a solid phase diffusion method in which bonding is effected at a high temperature under direct compression or through an insert. When a ceramic material is bonded to a metal according to the solid phase diffusion method, a flexural bonding strength of at least 100 MPa and occasionally 500 MPa (which is equal to the strength of the ceramic material per se) is obtained, and the residual stress generated by the difference of the thermal expansion coefficient is moderated by an insert interposed between the ceramic material and metal. Accordingly, the characteristics of the ceramic material and metal can be exerted most effectively in the solid phase diffusion method. Nevertheless, in the solid phase diffusion method, since a hot isostatic pressing (HIP) or a hot pressing (HP) must be used, the cost is increased. According to the solder bonding method, a metal layer is formed between a ceramic material and an adherend and the assembly is heated to fuse the metal layer and effect bonding. The strength and air tightness of the bonded portion are excellent and the bonding operation can be easily accomplished. In the case where a ceramic material is bonded to a metal or ceramic materials are bonded together according to this method, the surface of the ceramic material must be metallized, and as the wetting property of this metallized layer with the ceramic material is high, the bonding work quantity of the interface is increased and a strong bonding layer can be obtained.

But, since silicon nitride has a strong covalent bonding property, the affinity with a melt, that is, the wettability, is low. Furthermore, silicon nitride has a low reactivity with various substances. Therefore, a good metallized layer cannot be formed, and it is difficult to bond a ceramic material to a metal or ceramic materials to each other easily and firmly.

In view of the foregoing defects of the conventional techniques, it is a primary object of the present invention to provide a bonding composition capable of bonding ceramic materials such as silicon nitride (Si₃ N₄) to each other or bonding a ceramic material to a metal easily and firmly, and a bonding method using this bonding composition.

DISCLOSURE OF THE INVENTION

In accordance with one aspect of the present invention, there is provided a bonding composition for bonding a ceramic material to an adherend, which comprises a metal oxide melt comprising (a) CaO, (b) at least one metal selected from the group consisting of SiO₂ and Al₂ O₃, and (c) at least one metal oxide selected from the group consisting of TiO₂, ZrO₂, Cr₂ O₃, MfO₂, Nb₂ O₅, and Ta₂ O₅.

In accordance with another aspect of the present invention, there is provided a method for bonding ceramics, which comprises melt-bonding the above-mentioned bonding composition to the surface of a ceramic material and bonding the ceramic material to an adherend, if necessary through at least one layer selected from the group consisting of a plating layer, a solder layer, and a buffer layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the state wherein ceramic materials are bonded together according to the bonding method of the present invention.

FIG. 2 is a sectional view showing the state wherein a ceramic material is bonded to a metal according to the bonding method of the present invention.

FIG. 3 is a sectional view showing the state wherein a ceramic material is similarly bonded to a metal according to the bonding method of the present invention.

FIGS. 4 and 5 are diagrams showing the results of SEM-EPMA analysis of the bonded portion formed by bonding ceramic materials to each other according to the bonding method of the present invention.

FIG. 6 is a diagram showing the results of EPMA analysis of the bonded portion.

BEST MODE FOR CARRYING OUT THE INVENTION

The bonding composition of the present invention is prepared by weighing, mixing, and pulverizing predetermined amounts of three starting components (a), (b), and (c), that is, (a) CaO, (b) SiO₂ and/or Al₂ O₃, and (c) at least one metal oxide selected from TiO₂, ZrO₂, HfO₂, Nb₂ O₅, Ta₂ O₅, and Cr₂ O₃, calcining the mixture at a high temperature of 1,300° to 1,500° C., and pulverizing the calcined product to form a fine powder having a uniform composition, or using the uncalcined mixture as the powdery composition. In order to form a uniform reaction phase on the surface of silicon nitride, preferably the fine powder is composed of fine particles having a particle size smaller than 44 μm.

The ratio of the three components (a), (b), and (c) constituting the metal oxide melt is preferably such that the metal oxide melt comprises 30 to 70% by weight of the component (a), 10 to 50% by weight of the component (b), and 5 to 60% by weight of the component (c).

In bonding ceramic materials such as silicon nitride to each other, or bonding a ceramic material to a metal by using the bonding composition of the present invention, the bonding composition (in the form of a fine powder) is first mixed with an (organic binder to form a paste. (Commercially available screen oil or ethyl cellulose is preferably used as the binder.) Then, the paste is coated or thick-printed in a predetermined thickness on the surface of silicon nitride (Si₃ N₄), and the applied paste is sufficiently dried at 100° to 200° C. and calcined at 1,300° to 1,600° C. for 10 to 60 minutes. As the bonding atmosphere, there are ordinarily used atmospheric pressure N₂ gas, H₂ -N₂ mixed gas, compressed N₂ gas, and inert gas (such as Ar gas). When silicon nitride articles are bonded together, an oxidizing atmosphere may be adopted. Note, the abovementioned calcination temperature and time were determined so as to obtain an air-tight bonding intermediate layer.

The present invention will now be described in detail with reference to embodiments wherein the CaO-SiO₂ -TiO₂ or CaO-Al₂ O₃ -TiO₂ system is used as a typical instance of the metal oxide melt (sometimes referred to as "solder" hereinafter).

In the case where the solder used in the bonding method of the present invention has a composition of CaO-SiO₂ -TiO₂, in order to prevent the occurrence of phenomena inhibiting the bonding of ceramics, caused by a chemical reaction in the bonding interface, such as blowing and uneven calcination, preferably the composition should comprise 30 to 60% by weight of CaO, 15 to 45% by weight of SiO₂ and 10 to 40% by weight of TiO₂. Namely, if the composition is within the above-mentioned range, the viscosity of the melt is low and a reaction gas generated during the chemical reaction described below is sufficiently discharged and transpired, and SiO₂ formed as a by-product is absorbed as a component of the melt. For example, it was confirmed that a composition comprising 35 mole % of CaO, 35 mole % of SiO₂, and 30 mole % of TiO₂ (30.4% by weight of CaO, 37.1% by weight of SiO₂ and 32.5% by weight of TiO₂) has a good wettability.

Namely, TiO₂ in the solder in the molten state at the calcination temperature undergoes the following dissociation:

    TiO.sub.2 →Ti.sup.4+ +20.sup.2-                     (1)

In this case, O²⁻ breaks the network structure of SiO₂ and makes no contribution to the chemical reaction. Ti⁴⁺ reacts in the interface with nitrogen in the nascent state, produced by decomposition of Si₃ N₄, to cause precipitation of TiN, as indicated below:

    Si.sub.3 N.sub.4 +4Ti→3Si+4TiN-126.8 kcal/mole      (2)

    3Si+30→3SiP (g)q                                    (3)

TiN thus precipitated is accumulated at a highest concentration in the interface of Si₃ N₄ but is little diffused in the interior of Si₃ N₄. SiO₂ formed as the by-product is absorbed in the solder while SiO is transpired.

Since TiN precipitated in the foregoing manner has a very high alignment with silicon nitride (β-Si₃ N₄), from the crystallochemical viewpoint, a very dense intermediate layer is formed. Accordingly, when the solder of the present invention is used for bonding, a very high bonding strength can be obtained. Note, FIGS. 4 and 5 show the results of SEM-EPMA analysis of the product obtained by bonding a ceramic article 1 to a ceramic article 2 according to the bonding method of the present invention. FIG. 6 shows the results of a similar EPMA analysis. It is seen that TiN coheres in the bonding interface 3 between ceramics (Si₃ N₄) 1 and 2 to form a bonding layer 4. When the accumulation degree of TiN in the bonding boundary of Si₃ N₄ is determined by analysis of the SEM-EPMA images shown in FIGS. 4 and 5, it is confirmed that 36.8% by weight of all of TiN present in the bonding layer 4 is accumulated in a thickness of 2 μm in the bonding interface 3 between ceramics (Si₃ N₄) 1 and 2. In this embodiment, the thickness of the bonding intermediate layer is 25 μm, and TiN is condensed at a concentration of at least 3 times in the boundary portion. Namely, in the present invention, the solder component such as Ti, Zr or Cr is concentrated and precipitated in the bonding boundary between ceramics (Si₃ N₄).

In the case where the metal oxide melt has a composition of CaO-Al₂ O₃ -TiO₂, in order to obtain a good bonding state, preferably the composition should comprise 30 to 70% by weight of CaO, 10 to 50% by weight of Al₂ O₃, and 10 to 40% by weight of TiO₂.

In the case of the composition of CaO-SiO₂ -TiO₂, the composition of the melt of the intermediate layer, from which TiN precipitated with the advance of the reaction in the interface of Si₃ N₄ is removed, contains SiO₂ at a content higher than in the starting composition and TiO₂ at a content lower than in the starting composition and the composition consists of calcium silicate titanate in which impurities in Si₃ N₄ have been absorbed. On the other hand, in the case of the CaO-Al₂ O₃ -TiO₂ system, the solder is transformed to a four-component composition including SiO₂ formed as the by-product, in addition to the above-mentioned three components.

From this fact, it is seen that the present invention can be applied to a composition comprising at least four components, formed by combining solder compositions comprising three components.

In the case where Si₃ N₄ (which comprises particles having a particle size smaller than 10 μm at a content of at least 97% by weight based on the total particles and has an α-content of at least 90% by weight) is added to the fine solder powder, the bonding strength can be further increased. This effect is observed when the composition comprises 90 to 15% by weight of the fine solder powder and 10 to 85% by weight of powder Si₃ N₄, and a highest effect is obtained when the content of the solder is about 60% by weight and the content of Si₃ N₄ is about 40% by weight, although the optimum ratio differs to some extent according to the composition of the fine solder powder. It is considered that by incorporating Si₃ N₄ into the bonding layer, the thermal expansion coefficient of the bonding layer is brought close to that of an Si₃ N₄ sintered body, and the residual strain in the bonding layer after the bonding is reduced.

In the case where bonding of a ceramic material such as Si₃ N₄ to a metal is intended, after coating and calcination of the solder paste, Ni is plated on the obtained layer to improve the wettability at the soldering, and bonding is accomplished by using a solder.

If a powder of a metal having a relatively high melting point, such as W, Mo, Nb, Cr, Ti, V, Zr or Mn or an alloy thereof is added when preparing the solder paste, the soldering strength after the metallization can be further increased. Although the amount of the metal added is changed according to the composition of the solder, the high-melting-point metal or alloy powder is generally added in an amount of 5 to 95% by weight based on the sum of the solder and the high-melting-point metal or alloy powder, and a highest effect is attained if the amount of the high-melting-point metal or alloy powder added is about 30% by weight.

In the case where a ceramic material such as Si₃ N₄ is bonded to a metal, there may be adopted a two-layer structure in which the bonding layer contiguous to Si₃ N₄ is composed of the solder and the layer contiguous to the metal is a mixture composed of at least one metal selected from the above-mentioned metals and the solder. In this embodiment, the solder paste is coated or thick-printed on the surface of Si₃ N₄ and dried at 100° to 200° C., a mixed paste of the fine solder powder and the high-melting-point metal is then coated, and the upper and two layers are integrally calcined.

In this embodiment, preferably the mixing ratio between the fine solder powder and the metal component is such that, as calculated in the state where the fine solder powder is molten, the content of the fine solder powder is 5 to 20% by weight and the content of the high-melting-point metal is 95 to 80% by weight. This mixing ratio is necessary and sufficient for filling spaces in the metal component with the melt of the solder during the high-temperature calcination and integrating the mixture layer with the solder melt of the layer contiguous to Si₃ N₄.

In this embodiment, as in the case of bonding Si₃ N₄ to Si₃ N₄, calcination is carried out at 1,300° to 1,600° C., preferably 1,450° to 1,550° C., for 10 to 60 minutes, normally in an atmospheric pressure N₂ atmosphere. In the case where ceramic materials are bonded together, calcination may be carried out in H₂ +N₂ mixed gas, inert gas (for example, Ar gas) or compressed N₂ gas, but in the case where Si₃ N₄ is bonded to a metal, an oxidizing atmosphere such as air is not suitable. Although the calcination temperature is changed to some extent according to the composition of the fine solder powder, the calcination temperature is ordinarily a temperature at which a melt is formed on the surface of Si₃ N₄, and the calcination time is a time necessary for forming an air-tight bonding intermediate layer by reaction of the melt with Si₃ N₄ on the surface of Si₃ N₄.

The present invention will now be described in detail with reference to the following examples.

EXAMPLE 1 Bonding of Ceramic Material (Si₃ N₄) to Ceramic Material (Si₃ N₄)

Calcium carbonate (CaCO₃), silicic anhydride (SiO₂), and titanium oxide (TiO₂) were weighed so that a composition shown in Table 1 was obtained, and were wet-pulverized for 3 hours in a centrifugal rotation type agate ball mill. The pulverization product was dried at 100° C. for 24 hours and the obtained fine solder powder was mixed with commercially available screen oil to form a solder paste. The solder paste was thick-printed printed on the surface of a ceramic article (Si₃ N₄) having a size of 15 mm×30 mm×5 mm to form an adhesive layer 11 of a solder of a metal oxide melt solder of CaO-SiO₂ -TiO₂. The sample was dried at 200° C. for 6 hours. Two of the ceramic articles (Si₃ N₄) 10 having the solder adhesive layer 11 were piled so that the solder adhesive layers are placed in close contact with each other as shown in FIG. 1. The assembly having a structure shown in FIG. 1 was calcined at a calcination temperature of 1,500° C. for 30 minutes in an atmospheric pressure N₂ atmosphere or compressed N₂ atmosphere. Note, in FIG. 1, reference numeral 12 represents a first supporting plate formed of carbon (graphite), reference numeral 13 represents a supporting plate formed of Al₂ O₃, reference numeral 16 represents a second supporting plate formed of Al₂ O₃, and reference numeral 14 represents an organic adhesive used for bonding the carbon (graphite) supporting plate to the ceramic article 10.

Several test pieces having a size of 3 mm×4 mm ×40 mm were cut from the calcined integrated sample, and the 4-point (three equal portions) bending strength test was carried out at a cross head speed of 0.5 mm/min and a span length of 30 mm. The obtained results are shown in Table 1. Note, the thickness of the bonding intermediate layer was 20 to 30 μm.

For comparison, ceramic articles (Si₃ N₄) were bonded by using an oxide solder (CaO-MgO-Al₂ O₃ system), and the strength test was carried out in the same manner as described above. The obtained results are shown in the lower portion of Table 1.

                  TABLE 1                                                          ______________________________________                                                        4-Point Bending Strength                                                       (kg/mm.sup.2)                                                   Paste Composition                                                                               Atmospheric Compressed                                        (% by weight)    Pressure    (5 kg/cm.sup.2)                                   Run No.                                                                               CaO    SiO.sub.2                                                                             TiO.sub.2                                                                            N.sub.2 Atmosphere                                                                       N.sub.2 Atmosphere                        ______________________________________                                         1      30     30     40    12.3      13.1                                      2      35     35     30    14.5      14.9                                      3      35     40     25    15.2      15.0                                      4      35     25     40    10.5      12.1                                      5      45     45     10    19.7      21.3                                      6      45     35     20    25.0      37.4                                      7      45     25     30    18.3      18.9                                      8      60     30     10    13.1      12.6                                      9      60     15     25    11.8      12.1                                      ______________________________________                                         Com-                       Atmospheric                                                                              Compressed                                parative                   Pressure  (5 kg/cm.sup.2)                           Run No.                                                                               CaO    MgO    Al.sub.2 O.sub.3                                                                     N.sub.2 Atmosphere                                                                       N.sub.2 Atmosphere                        ______________________________________                                         1      20     30     50    7.5       7.2                                       2      30     60     10    6.8       7.3                                       3      50     10     40    4.8       5.4                                       ______________________________________                                    

EXAMPLE 2 Bonding of Ceramic Material (Si₃ N₄) to Ceramic Material (Si₃ N₄)

To the fine solder powder of Run No.

6 in Table 1 of Example 1 was added 10 to 80% by weight of powdery α-Si₃ N₄ containing particles having a particle size smaller than 10 μm in an amount of at least 97% by weight based on the total particles, and a past was prepared from the mixture. In the same manner as described in Example 1, calcination was carried out at 1,500° C. for 30 minutes in an atmospheric pressure N₂ atmosphere. After the calcination, test pieces having a size of 3 mm×4 mm×40 mm were cut from the sample and the strength test was carried out in the same manner as described in Example 1. The obtained results are shown in Table 2.

                  TABLE 2                                                          ______________________________________                                                Solder/Si.sub.3 N.sub.4 Weight                                                                4-Point Bending                                                 Ratio in Paste Strength (kg/mm.sup.2)                                   Run No.  Solder    Si.sub.3 N.sub.4 Powder                                                                   N.sub.2 Atmosphere                               ______________________________________                                         2-1      90        10         25.5                                             2-2      70        30         26.7                                             2-3      60        40         28.5                                             2-4      50        50         27.3                                             2-5      40        60         26.0                                             2-6      20        80         25.2                                             ______________________________________                                    

EXAMPLE 3 Bonding of Ceramic Material (Si₃ N₄) to Metal

Calcium carbonate (CaCO₃), silicic anhydride (SiO₂) and titanium oxide (TiO₂) were weighed so that a composition of Run No. 6 in Table 1 of Example 1 was obtained, and the mixture was melted at 1,500° C. for 60 minutes in a platinum crucible and wet-pulverized for 3 hours in an agate ball mill to obtain a fine solder powder. The fine powder was mixed with screen oil to form a paste. As shown in FIG. 2, the paste was coated on the surface of an article 15 of Si₃ N₄ having a size of 15 mm×30 mm×5 mm and was dried at 200° C. for 3 hours. Then, a mixed paste comprising the above-mentioned fine solder powder and a powder of a metal or alloy of W, Mo, Cr, Ti, W-Mn, Mo-Mn or Mo-Cr was coated on the above-mentioned paste layer to form a metallized adhesive layer 28, and the coated layer was dried at 200° C. for 3 hours and calcination was carried out at 1,500° C. for 30 minutes in an atmospheric pressure N₂ atmosphere. An electroless Ni plating layer 17 having a thickness of about 2 μm was formed on the so-obtained metallized adhesive layer 28. Two silver solder (BAg-8) layer 18 specified in JIS (Japanese Industrial Standard) and each having a thickness of 0.1 mm, were formed on and below a buffer member 19 formed of a Cu plate having a thickness of 2 mm. The silver-coated Cu buffer member was placed on the plating layer 17. A plate 20 of S45C steel was further piled and heat soldering was carried out at 790° C. for 10 minutes. The results of the shear strength test are shown in Table 3.

                  TABLE 3                                                          ______________________________________                                                       Solder/Metal                                                     Metal         Ratio in Paste                                                          Powder and Solder             Shear                                            Composition                                                                               (% by    Metal Powder                                                                             Strength                                  Run No.                                                                               (weight ratio)                                                                            weight)  (% by weight)                                                                            (kg/mm.sup.2)                             ______________________________________                                         3-1    W           5       95        2.91                                      3-2    W          10       90        4.01                                      3-3    W          20       80        3.70                                      3-4    Mo         10       90        4.13                                      3-5    Cr         10       90        4.95                                      3-6    Ti         10       90        5.01                                      3-7    W--Mn (8:2)                                                                               10       90        6.51                                      3-8    Mo--Mn (8:2)                                                                              10       90        6.48                                      3-9    Mo--Cr (8:2)                                                                              10       90        6.21                                      ______________________________________                                    

EXAMPLE 4 Bonding of Ceramic Material (Si₃ N₄) to Metal

Calcium carbonate (CaCO₃), silicic anhydride (SiO₂) and titanium oxide (TiO₂) were weighed so that a composition of Run No. 6 in Table 1 of Example 1 was obtained, and the powdery mixture was melted at 1,500° C. for 60 minutes in a platinum crucible and wet-pulverized for 3 hours in an agate ball mill to obtain a fine solder powder. To the powder was added 5 to 50% by weight of a metal powder of V, Zr, Nb or W-Mn, and the mixture was mixed with screen oil to form a paste. As shown in FIG. 3, the paste was coated on the surface of an article 21 of Si₃ N₄ having a size of 15 mm×30 mm×5 mm and dried at 200° C. for 3 hours. Then, calcination was carried out in atmospheric pressure N₂ gas at a temperature suitable for the composition, which was selected in range of from 1,450° to 1,600° C.

An Ni plating layer 23 having a thickness of about 2 μm was formed on the so-obtained metallized adhesive layer 22 comprising the CaO-SiO₂ -TiO₂ solder as the metal oxide and the metal powder. Silver solder (BAg-8) layers 24 specified in JIS and each having a thickness of 0.1 mm, were formed on and below a buffer member 25 composed of a Cu plate having a thickness of 2 mm. The silver-coated Cu buffer member was placed on the plating layer 23. Then, an S45C steel plate 26 was further piled and heat soldering was carried out at 790° C. for 10 minutes. The results of the shear strength test are shown in Table 4.

                                      TABLE 4                                      __________________________________________________________________________     Metal Powder  Solder/Metal Ratio in Paste                                                                    Calcination                                                                           Calcination                                                                          Shear                                    and Composition                                                                         Solder  Metal Powder                                                                           Temperature                                                                           Time  Strength                            Run No.                                                                             (weight ratio)                                                                          (% by weight)                                                                          (% by weight)                                                                          (°C.)                                                                          (minutes)                                                                            (kg/mm.sup.2)                       __________________________________________________________________________     4-1  V        90      10      1,450  30    4.95                                4-2  Zr       90      10      1,450  30    5.27                                4-3  Nb       90      10      1,600  60    4.78                                4-4  W--Mn (8:2)                                                                             95       5      1,500  30    3.47                                4-5  W--Mn (8:2)                                                                             90      10      1,500  30    4.57                                4-6  W--Mn (8:2)                                                                             70      30      1,500  30    5.48                                4-7  W--Mn (8:2)                                                                             50      50      1,500  30    4.21                                __________________________________________________________________________

EXAMPLE 5 Bonding of Ceramic Material (Si₃ N₄) to Metal

With respect to the sample of Run No. 3-7 in Table 3 of Example 3, metallization and plating were carried out in the same manner as described in Example 3. Silver solder (BNi-5) layers specified in JIS and each having a thickness of 0.1 mm, were formed on and below a buffer member composed of an Ni plate having a thickness of 2 mm. The silver-coated Ni buffer member was placed on the plating layer, and a plate of S45C steel was piled thereon. Heat soldering was carried out at 1,200° C. for 10 minutes. The hot shear strength test of the bonded structure was carried out at normal temperature, 500° C., 700° C., and 850° C.. The obtained results are shown in Table 5.

For comparison, metallization was carried out by using a silver-copper eutectic solder comprising 70% by weight of Ag, 20% by weight of Cu, and 10% by weight of Ti, and a bonded structure was prepared in the same manner as described above. The hot shear strength test of the bonded structure was carried out at the above-mentioned temperatures. The obtained results are shown in Table 5.

                  TABLE 5                                                          ______________________________________                                                      Shear Strength (kg/mm.sup.2)                                      Test Temperature (°C.)                                                                 Present Invention                                                                           Comparison                                         ______________________________________                                         Ambient temperature                                                                           6.61         7.21                                               500            5.80         5.04                                               700            4.71         2.57                                               850            4.00         Measurement                                                                    impossible                                         ______________________________________                                    

CAPABILITY OF EXPLOITATION IN INDUSTRY

In the bonding composition of the present invention, a metallized layer is formed by the metal oxide melt and bonding is accomplished by heat-melting the metallized layer. Accordingly, the metallized layer can be formed in a good state on the surface of a ceramic material having fine pores and a low wettability and, therefore, bonding of ceramic materials to each other or bonding of a ceramic material to a metal can be easily accomplished by the soldering method.

Furthermore, TiO₂ in the metal oxide melt precipitates TiN at the calcination temperature, and this TiN is accumulated at a high concentration in the interface of the ceramic material (Si₃ N₄) to form a dense intermediate layer. Therefore, ceramic materials (Si₃ N₄) can be bonded together, or a ceramic material (Si₃ N₄) can be bonded to a metal. 

We claim:
 1. A method for bonding ceramics, which comprises melt-bonding a metal oxide melt bonding composition consisting of, based on the weight of the metal oxide melt, (a) 30 to 60% by weight of CaO, (b) 15 to 45% by weight of SiO₂ and (c) 10 to 40% by weight of TiO₂ on the surface of a ceramic material composed of Si₃ N₄ ; and bonding the ceramic material to an adhered composed of Si₃ N₄.
 2. A bonding method according to claim 1, wherein the metal oxide melt incorporates a high-melting-point metal or high-melting-point alloy and the amount of the high-melting-point metal or high-melting-point alloy incorporated is 5 to 95% by weight based on the sum of the metal or alloy and the metal oxide components (a), (b) and (c).
 3. A bonding method according to claim 2, wherein the high-melting-point metal or high-melting-point alloy is at least one member selected from the group consisting of W, Mo, Nb, Cr, Ti, V, Zr, Mn and alloys thereof.
 4. A bonding method according to cliam 2, wherein the high-melting-point metal or high-melting-point alloy is a metal mixture or alloy comprising 75 to 95% by weight of W and 5 to 25% by weight of Mn.
 5. A bonding method according to claim 2, wherein the high-melting-point metal or high-melting-point allow is a metal mixture or alloy comprising 75 to 95% by weight of Mo and 5 to 25% by weight of Mn.
 6. A method for bonding ceramics, which comprises the steps of:melt-bonding a bonding composition comprising, based on the weight of the bonding composition, 15 to 90% by weight of a metal oxide melt and 10 to 85% by weight of silicon nitride (Si₃ N₄); said metal oxide melt consisting essentially of, based on the weight of the metal oxide melt, (a) 30 to 60% by weight of CaO, (b) 15 to 45% by weight of SiO₂ and (c) 10 to 40% by weight of TiO₂ on the surface of a ceramic material composed of Si₃ N₄ ; and bonding the ceramic material to an adhered composed of Si₃ N₄.
 7. A method for bonding ceramics, which comprises melt-bonding a bonding composition comprising (1) 5-95% by weight, based upon the total weight of the bonding composition, of a metal oxide melt consisting essentially of, based on the weight of a metal oxide melt, (a) 30 to 70% by weight of CaO, (b) 10 to 50% by weight of at least one oxide selected from the group consisting of SiO₂ and Al₂ O₃ and (c) 5 to 60% by weight of at least one metal oxide selected from the group consisting of TiO₂, ZrO₂, Cr₂ O₃, HfO₂, Nb₂ O₅ and Ta₂ O₅, and (2) 5 to 95% by weight, based on the total weight of the bonding composition, of a high-melting-point metal mixture or high-melting-point allow comprising 75 to 95% by weight of W or Mo and 5 to 25% by weight of Mn, on the surface of a ceramic material composed of Si₃ N₄ ; and bonding the ceramic material to an adhered composed of Si₃ N₄.
 8. A bonding method according to claim 7, wherein the bonding composition further comprises silicon nitride (Si₃ N₄) at a mixing ratio such that the content of the metal oxide melt is 15 to 90% by weight and the content of Si₃ N₄ is 10 to 85% by weight. 